1. Field of the Invention
The invention relates to a method and device for the automated application of a bead of adhesive to a joining surface wherein the distance between an adhesive application nozzle and the joining surface is measured with contactless measurement.
2. Description of Prior Developments
A method for the automated application of adhesive, sealing compounds, etc. to a joining surface is known, for example, from the generic DE 930 38 57 U1. In this method, an approximately planar joining surface is guided past the adhesive-application nozzle 9 at a predetermined distance from the outlet opening. Furthermore, electromagnetic distance-measuring systems for guiding welding and cutting machines at a predetermined distance from a metallic surface are known from U.S. Pat. Nos. 3,153,109, 3,217,204, 3,596,050, DE 1 941 728 and DE 28 29 851.
In the automotive sector, in particular in the body shell sector, particularly high demands are placed on adhesive bonds with regard to structural strength and sealing requirements. To produce a sealed, cleanly positioned adhesive bond, in addition to accurate metering of the amount of adhesive, the positioning of the bead of adhesive in particular plays an important role. While the amount of adhesive can be determined and monitored highly accurately with the aid of a metering control system, exact positioning of the bead of adhesive requires the distance between the application nozzle for the bead of adhesive and the joining surface to be maintained very accurately.
If the distance between the outlet opening and the joining surface is too great, considerable fluctuations in the position of the bead of the adhesive on the joining surface occur, so that it is impossible to achieve a well-defined positioning of the bead of the adhesive; on the other hand, if the distance between the outlet opening and the joining surface is too short, the bead of adhesive is squashed and loses its original round profile. A process which reliably maintains the optimum distance between outlet opening and joining surface during the automated application of a bead of adhesive to an undulating, three-dimensional joining surface which is subject to manufacturing tolerances can only be achieved if the process is accompanied by measurement of the distance between the application nozzle and joining surface and if the measured value thus obtained is used to control the distance.
The invention is therefore based on the object of proposing a method and a compact device with which beads of adhesive can be applied in a clean, reproducible manner to a joining surface.
An adhesive head includes a sensor which measures the distance of the outlet opening from the joining surface and compares this distance with a desired value. With the aid of the device according to the invention, it is possible to apply beads of adhesive with a high level of accuracyxe2x80x94irrespective of fluctuations in the component geometryxe2x80x94even to workpieces which are of complicated shape, while achieving a 100% seal in the adhesive bond and a high strength in particular on structural components. This leads to a reduction in scrap and therefore to a considerable cost saving compared to conventional adhesive systems. The highly accurate, reproducible positioning of the bead of adhesive with the aid of the method according to the invention moreover allows the adhesive-bonding technique to be employed in bodywork part areas which are difficult to access. Furthermore, the reliability of the process for applying the bead of adhesive allows this method to be used for the adhesive bonding of workpieces which are relevant from a safety aspect, in particular for the adhesive bonding of rebate edges, on which high demands are placed both with regard to strength and with regard to the seal provided.
To achieve an accurate, reproducible measurement result, it is expedient to use an inductive sensor. Unlike a capacitive sensor, which may supply incorrect measured values and therefore may result in an incorrect distance measurement if, for example if the nozzle becomes smeared, an irregular amount of adhesive passes between joining surface and sensor, and which therefore is of only limited suitability for distance measurements in adhesive applications, this inductive sensor still allows highly accurate, reproducible measured values to be obtained even if it is contaminated by adhesive.
The sensor and application nozzle are expediently mounted together on the movable carriage of a displacement unit, with the aid of which they can be displaced together approximately perpendicular to the joining surface. The actual distance of the sensor from the component surface, corresponding to the measurement signal, is compared with an adjustable set distance and the carriage is moved, as a function of the difference, in such a way that the distance between the application nozzle and the joining surface approximately corresponds to the desired distance throughout the entire adhesive-bonding operation.
Particularly accurate control of the distance between the application nozzle and joining surface is achieved if the distance sensor is fixedly connected to the application nozzle and is situated close to the outlet opening. The proximity of the sensor to the joining surface in this case results in a signal which is particularly sensitive with regard to distance and enables a direct conclusion to be drawn as to the position of the outlet opening with respect to the joining surface, thus ensuring rapid feedback. Furthermore, this allows the adhesive-bonding device to be of compact design. It is thus possible to apply beads of adhesive even along complicated paths, requiring multidimensional displacement and pivoting of the adhesive-bonding device with respect to the joining surface, without any collisions. In order in this case xe2x80x94irrespective of the curve of the path and direction of displacement of the adhesive-bonding device with respect to the joining surfacexe2x80x94to ensure a uniform measurement signal from the sensor, the sensor is advantageously designed in such a way that it surrounds the application nozzle in the form of a ring.
For the automated application of a bead of adhesive to a workpiece of complicated shape, it is advantageous to move the workpiece with respect to the adhesive-bonding device along the desired path of adhesion with the aid of a NC manipulator. In the process, the manipulator positions the workpiece in such a way that the (fixed) direction of displacement of the adhesive-bonding device at all times is approximately perpendicular to that area of the joining surface which lies opposite the application nozzle. The measured values from the sensor are thus used to rapidly control the vertical distance between the joining surface and the outlet opening and thus compensate for tolerances in the workpiece, while the path of the workpiece in the horizontal direction, which is less relevant in terms of accuracy, is determined by the manipulator.